This project involves studies of motor control in subhuman primates carrying out learned movements and in healthy human subjects and patients with parkinsonism, Huntington's disease, and tardive dyskinesia. The experiments on subhuman primates involve recordings of cell discharge in motor cortex and are designed to test the hypothesis that even for very small movements, neurons in motor cortex utilize a very wide dynamic range. It is proposed that high accuracy for small movements is based on existence within motor cortex of a "movement pattern generator" operating in concert with a "movement extent generator." The movement pattern generator would always use its full dynamic range, and would be responsible for both the temporal properties (onset time, velocity, cessation time, etc.) as well as the directional properties of the movement. The movement extent generator would correspond to a variable gain amplifier which would vary the magnitude of motoneuronal responses to the output of the pattern generator. Preliminary results are consistent with this hypothesis. Studies in human subjects are aimed at understanding the way in which set and expectancy modify motor processes in normal subjects and patients with psychomotor disorders listed above. These studies of motor processes in man have dealt with reflexes and intended movements occurring in response to externally produced arm displacements.